In radiation treatment planning, using either intensity-modulated radiation therapy (IMRT) or volumetric modulated arc therapy (VMAT) it is difficult to create satisfactory treatment plans for target volumes with concave parts. More specifically, healthy tissue in the vicinity of those concave parts of the target volume is unnecessarily irradiated. In addition, the geometric shapes of the irradiation fields may be more complicated for target volumes with concave parts. The plans can be manually improved by adding dummy risk volumes at those locations of the healthy tissue where irradiation is too high. Alternatively the plans can be manually improved by using a higher number of treatment beams that irradiate different parts of the target volume. This can be achieved either by limiting the leaf travel of the multi-leaf collimator (MLC) leaves for each beam or by manually setting the jaws of the MLC to restrict leaf motion of the beams.
An object of the invention therefore is to provide a method of determining a trajectory of a treatment device for radiation therapy which automatically seeks to avoid irradiation of tissue lying outside the target region while allowing for an optimal coverage of the target region with ionising treatment radiation.
Aspects of the present invention, examples and exemplary steps and their embodiments are disclosed in the following. Different advantageous features can be combined in accordance with the invention wherever technically expedient and feasible.